1. Field of the Invention
This invention relates to electro-optic (EO) sensors for image formation, and more particularly to a digitally scanned multi-cell EO sensor that provides capabilities similar to those of a dual-mode EO sensor.
2. Description of the Related Art
Many guided munitions (e.g. self-propelled missiles, rockets, gun-launched projectiles or aerial bombs) use a dual-mode EO sensor to guide the munition to its target. In a semi-active laser (SAL) mode, the sensor detects active guidance radiation in the form of laser radiation from a SAL designator that is reflected off of the target and locks onto the laser spot to provide line-of-sight (LOS) error estimates at an update rate required by the guidance system. In a passive imaging mode, the sensor detects IR radiation emitted from or reflected off of the target. The sources of IR energy are not artificial; they typically follow the laws of Planck radiation. The source may be the blackbody radiation emitted by the target directly or may, for example, be sunlight that is reflected off of the target. The passive imaging mode is typically used mid-flight or at the end of flight to process a more highly resolved image to determine whether or not the target is of interest or to choose a particular aimpoint on the target. The passive imaging mode operates at a much higher spatial resolution than the SAL mode. The passive imaging mode may be used to provide LOS error estimates to track the target when SAL designation is not available. However, due to its much higher spatial resolution, hence fewer incident photons per pixel, the passive imaging mode does not have the sensitivity to acquire and track the target at long ranges at the desired update rate.
A dual-mode EO sensor comprises a primary optical element having a common aperture for collecting and focusing SAL laser radiation and passive imaging radiation. A secondary optical element separates the SAL laser and passive imaging radiation by spectral band and directs the SAL laser radiation to a SAL detector and directs the passive imaging radiation to an IR imaging detector. The standard SAL laser designator produces laser radiation at 1.064 microns in the Near IR. The optics spatially encode an angle of incidence of the SAL laser radiation (e.g. a laser spot) at an entrance pupil onto the SAL detector. A quad-cell photodiode provides sufficient resolution to determine the LOS error estimate. The passive imaging radiation from a typical target is at long range, such that the electromagnetic wavefront at the sensor is considered to be composed of planar wavefronts. The structure of the target is imprinted on the composite wavefront as a summation of planar wavefronts with different slopes. The optics convert these slopes to spatial offsets in the image plane to form an image of the target on the pixelated IR imaging detector.
The IR imaging detector typically operates in the Short-Wave Infrared (SWIR) (1-2.5 um), Mid-Wave Infrared (MWIR) (3-5 um), or Long-Wave Infrared (LWIR) (8-14 um) electromagnetic radiation bands. With currently available technologies such as opto-mechanical scanning, staring focal plane array (FPA) or digital scanning (known as a “Rice pixel”), this detector may exhibit an effective spatial resolution, for example, of anywhere from 32×32 to 4,000×3,000 pixels. Selection of the desired band(s) for the passive imaging sensor depends on the target of interest and the expected atmospheric absorption bands. The SWIR Band is typically used in night conditions to provide high contrast. The MWIR band is selected if the expected targets are relatively hot (e.g. planes, missiles, etc.). The LWIR band is typically used to image targets that have operating temperatures slightly above the standard 300K background.
To provide the desired spatial resolutions for the IR imaging detectors, early systems utilized an opto-mechanical scanning architecture. These systems could scan a limited FOV across a larger field of regard (FOR) and stitch together an image that had the desired resolution. In some systems, a spinning mass gyro platform effectively scanned a single pixel or line of pixels across a wide FOR. The optical telescope itself is spun and precesses to map out a large scan area. These systems can achieve an extremely narrow point spread function. In a situation with a static scene and no constraints on frame rate across the FOR opto-mechanical is still the optimum system design as far as optical performance is concerned. However, in a dynamic scene that requires feedback at a particular update rate these systems are less than ideal. If a target is moving quickly through a scene, the act of scanning through that scene could cause the system to miss the target entirely.
As manufacturing techniques progressed and more and more pixels could be processed on a detector, the need to utilize opto-mechanical scanning prisms or spinning mass systems diminished. The system performance gains of being able to “see” a larger FOV instantaneously outweighed cost and optical performance constraints. These systems, typically referred to as staring focal plane arrays (FPAs), are the current standard in the electro-optic sensor industry. Staring FPAs provide the requisite resolution and FOV required. The staring FPA enables interrogation of different areas of the FOV concurrently at the native resolution of the FPA at high update rates. If a larger FOR is demanded for a particular application, a gimbal platform is typically used to move the FOV in a step-stare fashion across the FOR. This approach strikes a balance of cost between the detector and the opto-mechanical platform.
More recently there has been some desire to utilized wide FOV optics and larger detector formats and or smaller detector pixels to achieve a fixed post electro-optic sensor that is stabilized electronically and meets a large range of FOR requirements. However, unlike the digital camera industry that utilizes Silicon detector technology in the visible band, the push to larger and larger format detector arrays in the SWIR, MWIR and LWIR bands have not been followed by a reduction in cost of the detector for infrared systems. As users demand more resolution across a wider FOV the detector cost, and cost to cool the detector, have become the most expensive component in the electro-optic sensor system.
In an attempt to address this problem a group of researchers at Rice University developed in effect a single pixel digital scanning system known as the “Rice pixel” [Duane et al., Single-Pixel Imaging via Compressive Sampling, IEEE Signal Processing Magazine, March 2008 (83-91)]. They developed a light modulation technique that orthogonally coded the signals present on a large field of view MEMs based SLM. In this way they could mathematically reconstruct the entire scene via the frame sequence of a single pixel. The SLM was a commercial digital micro-mirror device (DMD) developed by Texas Instruments, Inc. for projection systems and thus was at much reduced cost to the typical analog opto-mechanical scanning systems. However, as the users of current electro-optic sensors have become accustomed to the performance provided by a staring FPA, those users have hesitated to move back to a single pixel architecture that demands significant processor resources, cannot maintain the high update rates and cannot concurrently interrogate different areas of the FOV.
The opto-mechanical scanning, staring FPA and digitally scanned Rice pixel are also deployed in single-mode passive IR imaging EO sensors in which SAL designation is not available. The opto-mechanical scanning and staring FPA single-mode systems must either accept a shorter acquisition range or provide large and expensive optical systems to increase sensitivity. The Rice pixel cannot instantly interrogate the entire FOV and has limited performance in dynamic scenes.